<!DOCTYPE html>
<html lang="zh-cn">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>

        Array.prototype.sum = function () {   //追加
            var sum = 0;
            for (let i = 0; i < this.length; i++) {
                sum += this[i];
            }
            return sum;
        }
        // Array.prototype = {                                  // 会覆盖
        //     sum: function () {
        //         var sum = 0;
        //         for (let i = 0; i < this.length; i++) {
        //             sum += this[i];
        //         }
        //         return sum;
        //     }
        // }

        var arr = [1, 2, 3];
        console.log(arr.sum());
        console.log(Array.prototype);
        var arr1 = new Array(11, 22, 33);
        console.log(arr1.sum());
    </script>
</body>

</html>